1. Field of the Invention
The present invention relates to a conductive brush charging device having a rotating conductive brush for charging to a given potential a photosensitive drum in an image forming device such as an electrophotographic printer.
2. Description Of the Related Art
Recent development of office automation has brought about a wide use of an electrophotographic image forming device such as a laser beam printer in computer output terminal equipment, facsimile equipment, copiers, etc. In such an image forming device, a photosensitive drum is charged to a given potential by a charger, and is next exposed to light according to image information to form an electrostatic latent image on the photosensitive drum. Thereafter, the electrostatic latent image on the photosensitive drum is developed with a toner to form a toner image, which is in turn transferred to a sheet of recording paper. The toner image transferred to the recording paper is next fixed to obtain a hard copy. After the transfer step, the photosensitive drum is de-electrified by an eraser, and the residual toner left on the surface of the photosensitive drum is scraped from the surface of the photosensitive drum by a cleaner, thus completing one cycle of print operation.
As the charger for charging the photosensitive drum surface to a given potential, a corona charger, a conductive roller charger, or a conductive brush charger is widely used. Of these chargers, the conductive brush charger has recently been noticed owing to its low cost, and the usability thereof is increasing. In an electrophotographic printer employing such a brush charger, a small amount of fine toner particles or fine additive particles yet remains on the photosensitive drum even after cleaning the residual toner from the photosensitive drum by using the cleaner.
The toner or additive left on the photosensitive drum after cleaning sticks to a rotating brush of the brush charger, causing a deterioration in charging characteristics of the brush charger to the photosensitive drum. As a result, there occurs a stain called "fog" in the background area of each printed sheet with an increase in number of printed sheets, thus reducing a print quality. Further, since the service life of the rotating brush of the conductive brush charger is relatively short, it is desired to achieve a method of effectively removing the toner or additive deposited to the rotating brush without reducing the service life of the rotating brush.
FIG. 9 shows a conductive brush charger 4 in the prior art. In the configuration shown in FIG. 9, a given voltage is applied to a rotating conductive brush 4a of the brush charger 4, so that the surface of a photosensitive drum 2 is charged to a given potential by sliding contact between the conductive brush 4a and the photosensitive drum 2. The photosensitive drum 2 is next exposed to light according to image information by an optical unit (not shown) to form an electrostatic latent image on the photosensitive drum 2. The electrostatic latent image is next developed with a toner to form a toner image, which is in turn transferred to a sheet of recording paper. The residual toner left on the photosensitive drum 2 after transferring the toner image is cleaned from the photosensitive drum 2 by a cleaner (not shown), thus completing one cycle of print operation.
In this manner, the residual toner on the photosensitive drum 2 is cleaned from the photosensitive drum 2 by the cleaner; however, a fine toner 3 in particular cannot be completely cleaned off by the cleaner and remains on the photosensitive drum 2 in some cases. This residual toner 3 on the photosensitive drum 2 sticks to the rotating conductive brush 4a in charging the photosensitive drum 2 with the brush charger 4. Although the amount of the residual toner 3 sticking to the conductive brush 4a is small in actual, the charging characteristics of the brush charger 4 to the photosensitive drum 2 are largely affected by the deposition of the residual toner 3. As a result, the charging characteristics of the brush charger 4 to the photosensitive drum 2 are reduced to cause the occurrence of fog in the background area of a printed sheet. An increase in number of cycles of print operation causes cumulation of the deposited toner on the conductive brush 4a, resulting in an increase in the fog occurring in the background area.
FIG. 10 shows another conductive brush charger 4' in the prior art intended to solve the above problem. The conductive brush charger 4' has a toner removing plate 5 kept in contact with a conductive brush 4a to scrape off a toner 3 deposited to the conductive brush 4a. The toner 3 removed by the toner removing plate 5 is stored into a toner receptacle 6. According to this configuration, the toner deposited to the conductive brush 4a can be removed by the toner removing plate 5, so that the charging characteristics of the conductive brush charger 4' to the photosensitive drum 2 can be improved to some extent. However, since the toner removing plate 5 is merely penetrated into the conductive brush 4a, the efficiency of removing the toner deposited to the conductive brush 4a is low. Accordingly, the charging characteristics of the brush charger 4' to the photosensitive drum 2 are reduced with the elapse of long time.
Other known conductive brush chargers will now be described.
Japanese Patent Laid-open No. Hei 3-288184 discloses a technique of rotating a developer removing member with vibration about the center of rotation of a rotary brush. However, the developer removing member partially removes a developer deposited to the tip of the rotary brush, and cannot completely remove the developer deposited to the brush. Japanese Patent Laid-open No. Hei 4-289878 discloses a conductive brush charger having a round rod for removing a toner deposited to a conductive brush. However, the toner once removed is possibly deposited again to the conductive brush, so it is difficult to efficiently remove the toner deposited to the conductive brush.
While other techniques are disclosed in Japanese Patent Laid-open No. Hei 4-366865 and No. Sho 63-221366, it is difficult to completely remove a toner deposited to a conductive brush according to these techniques, and there remains the problem that the charging characteristics of the brush charger to the photosensitive drum are reduced with the elapse of time.
As mentioned above, in the conventional conductive brush chargers, the toner deposited to the conductive brush cannot be completely removed. As a result, the repetition of print cycles over a long period of time causes cumulation of the toner deposited to the conductive brush, reducing the charging characteristics of the conductive brush charger to result in the occurrence of fog in the background area of each printed sheet.